Structural adhesive bonding primers serve three basic purposes: (1) they protect the adherend surface from being affected by the workshop environment; (2) they inhibit corrosion of the bonded surface during its service life; and (3) they provide a compatible surface to which the adhesive can bond for long-term strength. They should have excellent mar resistance, protect the adherend from re-oxidizing, and be readily cleaned prior to bonding using standard workshop procedures. Additionally, they should protect the adherend in a bonded structure during high humidity conditions; from salt corrosive environments; and withstand the effects of numerous fuels, hydraulic fluids, and lubricating oils. Standard industry tests include hundreds of hours at elevated temperatures or at 100% relative humidity or at exposure to salt fog (100% relative humidity and 95.degree. F. (35.degree. C.)) environment. Exposure tests for several days, both at ambient and elevated temperatures, to various fluids and chemicals are common modes of evaluation by industry. The primers should not adversely affect the performance of the bonding adhesive. Typical industry requirements for the combination of primer and adhesive are tensile shear strengths up to 41.4 Mpa (6,000 psi), peel strengths up to 11.34 N/mm (65 pounds per lineal inch), and long-term service (up to 6,000 hours) at elevated temperatures up to 450.degree. F. (232.degree. C.). These prerequisites for a suitable adhesive bonding primer must be generally met before the primer achieves commercial acceptance.
The corrosion resistance required of a structural adhesive bonding primer is quite high, particularly when compared to a coating composition that is applied to a substrate for non-structural purposes. A structural adhesive bonding primer is part of a composite structure. It is the first layer applied to the metal adherend. The next layer is the adhesive. It is put on the adherend for the purpose of joining an adhering surface to the adherend. This results in the formation of a structural composite.
The criticality of stress corrosion of structural adhesive bonds is the subject of Bascom, Adhesives Age, pages 28, 29-35, April 1979. In this article, the author notes:
The most severe limitation to the use of structural adhesives is the susceptibility of the bond lines to attack by moisture. The effect of the moisture is generally considered. a corrosion of the metal adherend, and evidence certainly exists to support this view. For example, in military operations in Southeast Asia, the seriousness of the problem was apparent from the extensive repair and refitting of aircraft caused by the delamination of aluminum skin and honeycomb structures. PA1 Presently, there is no clear understanding of the mechanisms involved in adhesive bond stress corrosion, nor are there any well established means of predicting bond durability under moist or wet environments. In fact, there is disagreement as to whether the primary attack is on the adhesive or the metal adherend. As for predicting bond lifetimes, there is no generally accepted test method for adhesive bond stress corrosion. PA1 Their high VOC contents (80-90%) are a target that Regional Air Quality Management Boards, especially those in areas prone to substantial periods of air pollution, are aggressively seeking to regulate. PA1 Waterborne industrial coatings are attractive because they usually contain only small amounts of solvent and can meet the newer air pollution regulations. In addition, they minimize fire and health hazards. On the other hand, aqueous systems lack the versatility and frequently the quality of solvent systems. Because of sensitivity to atmospheric conditions, they often must be applied under stringent controlled conditions of suitable temperature and humidity. Problems of corrosion often necessitate the use of stainless steel equipment. Some problems can be met by careful choice of solvents used in most waterborne coatings. PA1 Waterborne coatings may be defined as coatings that contain water as the major volatile component and that utilize water to dilute the coating to application consistency. These coatings consist mainly of resinous binder, pigments, water, and organic solvent. The type of pigmentation and the method of incorporation of the pigment vary widely. It is usually easier to incorporate pigments directly into the organic phase where conventional dispersion techniques can be applied . . . PA1 Waterborne coatings can be made by dispersing or emulsifying the resin binder by use of added surfactants. This technique leads to opaque liquids. Because some hard resins are difficult or impossible to disperse directly into water, the resin sometimes can be dissolved in a water-immiscible solvent, and the resulting solution dispersed by the use of added surfactants. In this case, the solvent aids subsequent film coalescence. Surface activity or water dispersability also can be introduced into resin molecules by chemical modification of the resin by functional polar groups such as the carboxyl group. PA1 Waterborne resin binders can be classified as anionic, cationic, or nonionic. Anionic dispersions are characterized by negative charges on the resin or by negative charges on the surfactant associated with the resin. Cationic dispersions have a positive charge on the resin or on the surfactant associated with the resin. Nonionic dispersions are those that have been dispersed by addition of nonionic surfactants or that contain a built-in hydrophilic segment such as polyethylene oxide which is part of the main chain of a relatively hydrophobic resin molecule.
The function of a structural adhesive bonding primer is to aid in keeping moisture from the adhesive-adherend interface and enhance the adhesion between the adherend and the adhesive. That action serves to minimize the impact of corrosion by acting as a barrier to moisture and passivating the adherend's surface from the impact of moisture that does penetrate to the adherend.
In the evolution of structural adhesive bonding primers, their formulations generally relied on dilute solvent solutions of modified epoxy or phenolic resins. These resins are generally considered innocuous, both being extensively used in food containers. However, materials used to cure these resins in adhesive bonding primers, such as amines, amides and imidazoles, may not be as innocuous. Solvents in the formulations have stimulated wide environmental concerns. The volatile organic compounds (VOCs) emitted by their evaporation from the adherend surface has been an ever increasing concern of industrial regulatory organizations.
Y. D. Ng and W. E. Rogers, in a paper entitled: "A Non-Chromated Water-Borne Adhesive Primer For Aerospace Applications" and given at the 33rd International SAMPE Symposium, during Mar. 7-10, 1988, discuss the environmental issues of adhesive primers. They point out that asbestos, at one time a favored raw material for adhesives, was virtually eliminated from the market since the early 1980's. They note that the aerospace industry has increased concern about using solvent-borne bonding primers.
Illustrative of this concern are the strict air quality requirements mandated by the South Coast Air Quality Management District..sup.1 Though Y. D. Ng and W. E. Rogers indicate that most solvent-borne adhesive primers have little difficulty complying with 1987 SCAQMD Rule 1124 VOC limit for adhesive primers at 850 grams/liter, they fail to point out that typical epoxy/phenolic solvent-based adhesive bonding primers at about 10 percent solids emit VOCs into the atmosphere at levels approaching 800 grams/liter. Such VOC levels have been accepted because of the high performance the solvent based primers bring to the application. With ever increasing environmental concerns, such VOC levels are becoming unacceptable and there is a strong demand for epoxy based adhesive bonding primers that accommodate environmental concerns. SCAQMD has set the VOC limit for adhesive bonding primers at 250 grams/liter minus water, starting Jan. 1, 1993. This accords with the trend set for the coatings industry. FNT .sup.1 South Coast Air Quality Management District (SCAQMD) has jurisdiction over air quality in the Greater Los Angeles Basis in southern California, U.S.A.
Such social reactions are stimulating the adhesive industry to find ways to reduce pollution by VOCs used as solvents in conventional adhesive bonding primers. Considerable emphasis exists to develop application technologies that reduce VOC emissions in adhesive bonding primer. A number of them have emerged to meet most but not all of the performance and application requirements, and at the same time meet emission requirements and regulations. One technology for overcoming the VOC problem involves the use of waterborne dispersions and solutions.
Clayton A, May, in his text entitled: EPOXY RESIN Chemistry and Technology, Second Edition, 1988, Published by Marcel Dekker, Inc., New York, N.Y., at page 766, makes the following characterization of waterborne coatings in general:
Some very finely dispersed resins appear as clear as [sic] slightly hazy liquids; they frequently are described as soluble, solubilized, colloidal dispersions, micro-emulsions, hydrosols, etc. These resins contain built-in functional groups that confer water "solubility" upon the resin, and, normally, external added surfactants are not used.
Waterborne dispersions and solutions are to be contrasted with the water containing emulsion systems (oil in water varieties). In the latter case, the emulsion particles contain a concentration of highly volatile, water immiscible solvent plus a surfactant that keeps the emulsified particles suspended in the continuous water phase. During application, they rely on solvents to coalesce the deposited emulsion particles coupled with the surfactant, in order to form a continuous film that is free of pin holes and other defects. The waterborne dispersions and solutions can effectively address the VOC problem as well as the structural bonding issues.
Waterborne structural adhesive bonding primers introduce entirely different selection of resin and cure system, and introduce formulation problems not dealt with in solvent based adhesive bonding primer systems. For example, waterborne adhesive bonding primers are not as resistant to corrosive environments as are the more conventional solvent-borne adhesives. The conventional epoxy resins used in solvent-based systems are not water soluble or effectively water dispersible.
As May, supra, points out, epoxy resins are suitably modified to convert them into waterborne coatings. They may be modified to make suitable waterborne adhesive bonding primers. Their modification involves the conversion of the epoxy resin into a molecule that contains enough anionic, cationic or nonionic water compatibilizing groups so as to make the resulting resin water dispersible or compatible, as desired. The term "water-dispersible, modified epoxy resin," as used herein and in the claims, means a resin that is derived from the use of at least one compound that contains a vicinal oxirane group in its manufacture and is rendered either cationic, anionic or nonionic so as to be dispersible in water to form a stable coatable film therefrom onto a solid substrate. Such a resin may be soluble or dispersible in water either in the form of an emulsion or as a discrete dispersion.
Y. D. Ng and W. E. Rogers, supra, discuss the development of a waterborne structural adhesive bonding primer that uses the same multi-functional epoxy novolac resin as was used in "Hysol's EA 9205R (a 350.degree. F.) [176.7.degree. C.] service, solvent-borne adhesive primer." The less polar epoxy groups on the resin were transformed into more polar hydroxyl moieties. "Further treatment produced the cationic salt of the resin which provided the desired solubility and physical property characteristics." A combination of inhibitors are mentioned as replacements for chromates. They are stated to be proprietary. Properties of the proposed adhesive primer are discussed.
Fan, in U.S. Pat. No. 4,355,122, patented Oct. 19, 1982, describes the manufacture of a special class of water-dispersible, modified epoxy resin. This water-dispersible, modified epoxy resin is a linear polymer and hence is thermoplastic. It is a member of the class of resins known as "phenoxys." Fan describes his special phenoxy resin in the Abstract as a "water-borne thermoplastic polyhydroxyether resins . . . prepared by grafting carboxyl-containing vinyl monomers onto phenoxy resins with a free radical initiator and then converting the graft copolymer to an ionomer suitable for use as coatings or adhesives." As Fan points out, the thermoplastic polyhydroxyether resin, i.e., the phenoxy resin, that is subjected to grafting is characterized by the formula: EQU --[--D--O--E--O--].sub.n --
"wherein D is the radical residuum of a dihydric phenol, E is an [sic] hydroxyl containing radical residuum of an epoxide and n represents the degree of polymerization and is at least 30 and is preferably 80 or more." The phenoxy resins are epoxy modified resins, as the Fan patent shows. The phenoxy resins are described in the patent as being the reaction product of about 0.985 to about 1.015 moles of an epihalohydrin ##STR1## where X is halogen, with one mole of a dihydric phenol together with from about 0.6 to 1.5 moles of an alkali metal hydroxide, such as sodium hydroxide or potassium hydroxide generally in an aqueous medium at a temperature of about 10.degree. C. to about 50.degree. C. until at least about 60 mole percent of the epihalohydrin has been consumed. In commercial phenoxy resins, the dihydric phenol is bisphenol A [2,2-bis(4-hydroxyphenol)propane] and the epihalohydrin is epichlorohydrin. The resulting polymer may have the formula: ##STR2## wherein either a or c is one or zero depending on whether bisphenol A or epichlorohydrin is used in molar excess, and b has the value of n defined above by Fan. The description of grafted phenoxy resins described at column 2, line 60 to column 7, lines 14, is incorporated herein by reference. Examples 1-10 of Fan are believed to illustrate preferred acrylic grafted resins that are rendered water soluble by converting the carboxy of the grafted acrylyl moiety to an hydroxyl amine salt by reaction with dimethylethylanolamine. A particularly preferred commercial resin of that class is UCAR Phenoxy Resin PKHW-35, sold by Union Carbide Chemicals & Plastics Company, Inc., Danbury, Conn. (U.S.A.), an amine-neutralized, carboxylated phenoxy resin that is colloidal in nature when dispersed in water. It would appear to be of that class of waterborne system that is characterized as a "microdispersion." It is characterized as having excellent emulsion stability from 0.degree. C. to 55.degree. C., without the need of a surfactant.
The performance advantages listed for UCAR Phenoxy Resin PKHW-35 are the following: excellent emulsion stability without added surfactant; hydroxyl and carboxyl functionality; "solution-type dry" behavior; high strength, stiffness, Tg, and ductility provided by the high molecular weight thermoplastic phenoxy backbone; tough and thermally stable films; crosslinks with standard melamine-formaldehyde resins to yield coating with superior properties, including outstanding impact resistance, flexibility, hardness, gloss, blush, and chemical resistance; highly compatible with a variety of thickeners and other system-enhancing additives; can be formulated as strippable coating; easy cleanup of processing equipment; and complies with FDA regulation 175.300 for food contact use. It is characterized as having excellent physical and chemical properties, and coupled with the excellent handling characteristics of this waterborne microdispersion, all of which makes the product, in the opinion of the supplie, a likely candidate for the following end-use: can coatings; coil coatings; wire coatings; specialty coatings; metal primers; laminating adhesives; flexibility modifier for rigid adhesives; and strippable coatings.
It is believed that grafting in PKHW-35 occurs at the tertiary hydrogen of the epichlorohydrin reaction product unit characterized in the phenoxy formula, as: ##STR3##
It is believed that PKHW-35 is made by grafting one or both of acrylic acid (or methacrylic acid) and methyl methacrylate as described in Example 5 of the Fan, U.S. Pat. No. 4,355,122, and worked up as described in Examples 6-10, except that the choice of solvent, conditions of reaction, concentrations, ratio of reactants are different in the manufacture of the commercial product.
Epoxy based adhesive bonding primers typically contain corrosion inhibitors. The most effective, and hence, the most widely used inhibitors are chromate (+6) salts such as potassium chromate, barium chromte, strontium chromate, zinc chromate and the like. They are usually part of the pigment composition of the formulation. Chromate pigments are listed as toxic substances under SARA Title III, Section 313..sup.2 They are listed as chemicals known to cause cancer or reproductive toxicity under California Proposition 65..sup.3 Their total or partial removal from any formulation is desirable, so long as the formulation possesses satisfactory corrosion inhibition. Due, in part, to this development, a need has arisen for non-chromate based corrosion inhibiting inhibitors. In particular, there is a need for non-chromate based corrosion inhibiting inhibitors for use in epoxy-derived adhesive bonding primers. FNT .sup.2 Superfund Amendments, a Re-Authorization Act of 1986 (SARA), Title III, Sections, 311, 312 and 313, United States Federal Regulation. FNT .sup.3 Proposition 65 (California Governor's list of "Chemicals known to cause cancer or reproduction toxicity"), State of California (U.S.A.) Regulation.
Corrosion occurs primarily as a result of electrochemical processes. At the interface between the substrate and the primer, an anodic surface is formed. Substrate ions, e.g. Fe.sup.0 and Al.sup.0 are oxidized, and go into solution. Electrons that are released from this reaction flow to the cathodic surface where they react with electrolytes such as water and salt solution. The formation of this circuit, and the subsequent chemical reactions, results in corrosion. The inhibitors act to prevent this corrosion through anodic passivation.
In order for anodic passivation to occur, the potential of the substrate must be increased enough such that substrate ions will not go into solution. The inhibitors work to cause this effect. Further, reduced pigment ions react with the soluble substrate ions to form an insoluble barrier on the substrate. This barrier prevents the movement of ions essential to the process of corrosion.
Chromates provide excellent corrosion protection and thus they are widely found in corrosion resistant adhesive primers. It follows that chromate replacements must approximate, or exceed, their performance. The industry typically assesses performance against standard testing procedures, such as ASTM B117-85.sup.4 which provides procedures for testing corrosion resistance in compositions deposited on a substrate..sup.5 These measurements may be utilized in conjunction with an industry standard such as BMS 5-89.20.sup.6 However, due to the critical nature of the adhesive components in structural performance of the article of manufacture in which it is employed, such as an airplane component, many manufacturers require even higher standards. FNT .sup.4 American Society for Testing and Materials, Philadelphia, Pa. .sup.5 With these standards as a guideline, the following test is a typical one to sample corrosion resistance of inhibitors. The substrate surface is cleaned and prepared. The primer is subsequently applied. The primer is applied at a thickness between 0.2 and 0.4 mils, with a preference for 0.3 mils. Once the primer is applied, it is cured at a suitable temperature. The substrate is cut into 3'.times.3' squares. A scribe is etched into the substrate in the shape of an "X." The scribes are subjected to a 5% sodium chloride salt spray for 1,000, 2,000, and 3,000 hours. At the end of these periods, the squares are removed and the excess salt is scrubbed loose. Three relevant measurements are taken: number of pits in the scribe, undercutting, and percentage of the scribe that is shiny. These measurements may be compared with an industry standard. These standards are set out in BMS5-89, infra. .sup.6 Boeing Material Specification.
As pointed out above, corrosion is a major issue with respect to structural adhesive primers. Consequently, a number of industries have strict corrosion inhibition standards when it comes to corrosion inhibition of structural adhesive bonding primers. The issue of corrosion inhibition is compounded in the case of waterborne structural adhesive bonding primers because of the formulation problems and the fact that they form a coating with different surface and structural characteristics.
The literature abounds with non-chromate corrosion inhibitors and many of them justly or unjustly are stated to be equivalent in performance to chromates. Typically, such claims are limited to solvent based coating systems and not to waterborne adhesive bonding primers where corrosion failure results in immediate structural delamination.
As noted above, the choice of resin for the manufacture of structural adhesive primers has been epoxy and phenolic resins. These resins are normally of low molecular weight therefore they possess a relatively high functionality to molecular weight ratio. Consequently, such resins end up producing cured products that have a high crosslink density. That crosslinked density contributes significantly to the solvent resistance, hardness, thermal resistance and other properties of the cured resins. The phenoxy resins are linear structures of relatively high molecular weight. They are thermoplastic. They can be made thermosetting because they possess functional hydroxyl groups along the backbone. However, such groups are secondary hydroxyl and they are within large bulky groups that can sterically hinder the reaction of the hydroxyl groups. Consequently, crosslinked phenoxy do not have comparable crosslinked densities to the conventional epoxy or phenolic resins. Phenoxy resins have not been a primary resin in aerospace adhesives. They may be used as an additive in conjunction with an epoxy resin but they do not replace the epoxy resin from the adhesive composition. It is not believed that phenoxy resins or waterborne versions thereof have been used in structural adhesive bonding primer compositions. Its properties are sufficiently different from those of epoxy and phenolic resins that they would not be regarded to have equivalent properties or be an obvious substitute for that application.